1. Field of the Invention
This disclosure relates to optical testing of integrated circuits and more particularly to optical interferometric probing of integrated circuits"" electrical activity.
2. Description of the Related Art
Electrical activity in integrated circuits can be monitored optically by probing changes in the refractive index and the absorption coefficient induced by varying bias voltages and carrier densities at semiconductor diode junctions. For example, changes in the refractive index and the absorption coefficient of the semiconductor material can be manifested by changes in the intensity of an optical beam reflected from the diode junction.
Paniccia et al. U.S. Pat. No. 5,872,360, incorporated herein by reference, discloses detecting an electric field in the active regions of an integrated circuit. In one embodiment, a laser beam is operated at a wavelength near the band gap of a semiconductor such as silicon. The laser beam is focused onto a P-N (diode) junction such as, for example, the drain region of an MOS transistor, through the back side of the semiconductor substrate. The beam passes through the junction, reflects off the oxide interface and metal behind the junction, and returns back through the junction and out of the silicon surface. Modulation of the electric field in the junction causes amplitude modulation in this reflected beam due to the combined effects of electro-absorption and electro-refraction.
Wilsher et al. U.S. Pat. No. 5,905,577, incorporated herein by reference, discloses dual laser beam probing of integrated circuits. A probe beam is used to sample the waveform on an integrated circuit device under test (DUT) during each cycle of a test pattern signal applied to the DUT. A reference laser beam is also used to sample the DUT at the same physical location sampled by the probe beam. Each reference measurement is made at a fixed time relative to the test pattern, while the probe measurements are scanned through the test-pattern, in the manner normal to equivalent time sampling, to reconstruct the waveform. For each test cycle, the ratio of probe and reference measurements is taken to reduce fluctuations in the probe measurement due to noise.
Changes in the refractive index and the absorption coefficient due to electrical activity in a DUT can also be manifested by phase modulation of an optical beam transmitted through or reflected by the DUT. Probes of phase modulation can be more sensitive than probes of reflectivity to electrical activity in a DUT. Heinrich et al. U.S. Pat. No. 4,758,092, incorporated herein by reference, discloses a method for interferometric measurement of phase modulation of an optical beam by an active semiconductor device. An optical beam is split into two beams which are focused on the DUT. One beam is focused on an active region, where it is phase modulated by a modulated refractive index, and the other beam is focused on an inactive region to provide a reference. The reflected beams are recombined and interfere. Modulation of the intensity of the interfering beams is attributed to modulation of the phase of the probe beam due to electrical activity in the probed region.
Typically, interferometric methods are extremely sensitive to vibration and to temperature changes. In the method of Heinrich et al. the relative phase of the two interfering beams is nominally insensitive to DUT vibration and temperature-induced movements in line with the beam axis. However, on a densely integrated circuit it is very difficult to find suitable points to place the reference beam. Also, there is still sensitivity to motion across the laser beam axis.
What is needed is an interferometric method for waveform probing of integrated circuits that is insensitive to vibration and temperature induced movements of the DUT, and that does not require finding a suitable reference point on the DUT near the active region of interest.
A method of detecting electrical activity in a semiconductor device as a repetitive electrical test pattern signal is applied to the device includes providing a first probe light pulse at a selected time during each repetition of the electrical test pattern, and providing a first reference light pulse at a time during each repetition of the electrical test pattern displaced relative to the selected time at which the first probe light pulse is provided. The first probe light pulse and the first reference light pulse are each divided to provide at least a second probe light pulse and a second reference light pulse. The first probe light pulse and the first reference light pulse are directed onto a region of the semiconductor device.
After the first probe light pulse and the first reference light pulse interact with the semiconductor device, the first probe light pulse is combined with the second probe light pulse to overlap in space and time, and the first reference light pulse is combined with the second reference light pulse to overlap in space and time. The overlapped probe light pulses are detected to provide a probe interference signal, and the overlapped reference light pulses are detected to provide a reference interference signal. The selected time is varied with the repetition of the electrical test pattern, The ratio of the probe interference signal and the reference interference signal is determined at a plurality of the selected times within the electrical test pattern.
An associated apparatus includes a source of a first probe light pulse, a source of a first reference light pulse, a splitter, a support for the semiconductor device, a beam combiner, a detector system, and a processor for determining a ratio of the probe interference signal and the reference interference signal.